The application relates generally to moisture sensors and in particular to moisture sensors for measuring water content in soil.
It is often desirable to detect the moisture content of a granular, particulate, fine or powdery medium. For example, in agriculture, it is often desirable to detect the moisture content of the soil in a region, so that irrigation or drainage systems can be controlled in accordance with the sensed moisture content. Soil moisture detection is also desirable for other purposes, e.g., for monitoring and evaluating the soil condition in construction sites, landscaped sites, mining operations, forest areas, flood control areas, or bio-remediation areas, such as areas in which liquid or other contaminant has spilled.
The ability to determine the moisture content of granular, particulate, fine or powdery media is also important in many other applications, such as cement or plaster making systems, gravel or brick processing systems, mined material moving or processing systems and food processing or handling systems, to name just a few. Another application for moisture content monitoring relates to the monitoring of the moisture content of certain water absorbing substrates, such as mushroom substrates, or other agricultural or laboratory substrates. As the demand for agricultural products increases and as the demand for higher quality products made from granular, particulate, fine or powdery media, or made with a moisture absorbing media increases, so does the demand for high quality moisture detection systems. Accurate moisture detection systems can dramatically increase the ability to meet the increasing market demands for these products. In addition, accurate moisture detection systems can increase the manufacturing efficiency and quality of such products.
For low dielectric media (such as soil), it has been recognized that moisture content affects the overall dielectric constant of such media to a detectable degree. For example, water has a relatively high dielectric constant of 80, while dry soil typically has a much lower dielectric constant of approximately 5 or 6. The water content in soil is, thus, generally a major contributor to the overall dielectric constant of the soil. An increase in the moisture content of soil will generally result in an increase in the dielectric constant of the soil. The same is typically true with many other low dielectric granular, particulate, fine or powder substances or media.
Accordingly, moisture sensors have been developed for reacting to the dielectric property of the medium being monitored. Moisture sensors of contemporary design typically employ a parallel plate capacitor configured to be immersed or embedded in the medium so that a portion of the medium becomes embedded between the parallel plates and functions as a dielectric between the plates. The capacitance provided by such a capacitor is used as part of an RC oscillator circuit having an oscillation frequency which varies with changes in the dielectric property of the small portion of the medium between the plates. The frequency of the oscillator circuit is used as an indicator of the moisture content of the medium.
The methods used to derive the capacitance from the frequency oscillator circuit in the prior art typically comprise either single frequency complex impedance calculation or sweeping the frequency to find the resonant point. In all the cases the frequency required for determining capacitance is very high in terms of MHz (e.g., 10 MHz). The high frequency requirement requires complex circuit design and/or costly components to implement.